FIG. 4 is a plan view showing a prior art bent waveguide for an optical integrated circuit.
In FIG. 4, reference numeral 1 designates a core region, and reference numeral 2 designate cladding regions provided at the both sides of the core region 1.
In this prior art bent waveguide, light which is incident from the left side in the drawing and advances toward the right side is radiated or scattered at the neighbourhood of the bent portion B.
FIG. 5 shows the transition of electric field distributions at each 10 micron interval in a case where a light having waveguide mode of TE.sub.0 is incident on the bent waveguide. In this case, refractive index n.sub.1 of core region 1 is 1-5, refractive index n.sub.2 of cladding region 2 is 1.49, and the wavelength is 1 micron. The electric field distributions after the light wave propagates through the bent portion B are gradually tapered toward outside, which means that the light wave is radiated. The coupling coefficient between the electric field distribution and TE.sub.0 waveguide mode after the light wave propagates through the bent portion B is about 37% due to the high rate of light wave radiation in the neighbourhood of the bent portion B.
In this prior art bent waveguide for an optical integrated circuit, light is radiated at a high rate in the neighbourhood of the bent portion B, thereby resulting in difficulty in realizing an optical integrated circuit.